Electric arc spraying is a thermal spray process in which one or more wires of either similar or dissimilar materials are melted, atomized and the molten particles are propelled onto a prepared surface building up a metallic coating. The thermal energy required to melt the wire is produced by an electric arc developed at the wire ends. A high velocity gas stream is used to atomize the molten metal in the arc and propel the fine droplets onto the surface to be coated. Arc spray equipment may be used to apply different types of coatings, for example corrosion protection coating, wear resistant coatings or resurfacing coatings.
An electric arc spray apparatus typically includes five major components: a wire feeder; a high current DC power source; a process controller; a source of high velocity gas; and an electric arc gun. The wire feeder continuously feeds one or more consumable metal wires at a uniform rate of speed. The wire is typically driven by a wire drive unit and then fed through insulated flexible conduits into wire guides and through electrode tips on an electric arc gun. The electrode tips generally guide the wires to an intersecting point where they meet.
Current from the DC power source is provided to the consumable wire at the electrode tips and an arc is formed where the two wires meet. The controller causes the power supply to furnish the correct electrical energy, wire feed speeds and gas flow.
Located directly behind the arc and positioned in a line with the arc is a nozzle through which an atomizing gas flows at high velocity. The atomizing gas is typically compressed air or an inert gas, such as argon or nitrogen. The velocity of the atomizing gas as it leaves the nozzle is typically up to thousands of feet per second and the velocity may be regulated over a broad range. The atomizing gas velocity has major effects on the characteristics of the coating.
The temperature of the arc may be up to the tens of thousands of degrees Fahrenheit. Because of these temperatures the particles when accelerated, impact and bond to the minute protrusions of a properly cleaned and roughened substrate, producing a high coating adhesion to the substrate and strong inner particle cohesive strength. In addition to strong bond strength, other advantages of electric arc spraying are low cost and ease of application relative to flame spraying.
Initially the ionized plasma in the arc is created as the two wires advance to an intersecting point and touch. A high density electric current is applied through the wires, thus creating extreme heat at the wires contact point, melting the touching portions of the metal wires and ionizing the surrounding gas. The ionized materials or plasma provides a relatively low resistance path for the flow of electric current. The high current flowing through the plasma and the voltage drop across the arc provide the necessary sustaining power to maintain the ionized state. The anode wire is heated by electrons which impact thereon after being released from the cathode wire surface. The cathode wire surface is heated by the impacting of positive gas ions.
For a particular application a given voltage, air velocity and current will be desired. In order to obtain the desired voltage the user often adjusts power supply voltage settings. The user also selects an air velocity. The current is usually selected by the user selecting a wire feed speed, because current is generally proportional to wire feed speed over most operating ranges. To assist in selecting the approximate wire feed speed, tables are often provided that give the relationship between wire feed speed and current for particular materials and operating conditions. To fine tune the current magnitude the user adjusts the wire feed speed.
Such prior art controllers generally have open loop control. Some prior art arc spray apparatus include a closed loop drive feeder speed control that monitors drive feeder speed and adjusts the speed of the drive motor to maintain a constant drive feeder speed. However, these controllers do not directly control the current in the arc. Thus, such controllers do not compensate for variations in operating conditions such as changes in the wire diameter, melting point of the wire, variations in the velocity of the air, variations in the alignment of the wire, or irregularities in the stiffness, cast or helix of the wire. Accordingly, a controller that compensates for such factors is desirable and would solve many of the problems of the prior art.
Generally speaking in electric arc spraying, conditions may be characterized by the operating voltage and current and gas flow rate. At certain voltage and current conditions an arc outage is likely to occur. At other voltage and current conditions a wire shortage is likely. At still other conditions the process can be performed smoothly, i.e., neither an arc outage nor a wire shortage is likely to occur. In the arc spraying process wire shortages and arc outages can lower the quality of the coating.
A wire shortage occurs when the wires are pushed too fast and touch, thereby bonding together. The bonded wires create a very low resistance path and current surges. After bonding together the wires travel together and eventually break off, but not necessarily at the tip. The high velocity air blows the end, called a spit, off, thus degrading the coated material. The spitting event is often repeated in a pulsing manner, wherein after the spit occurs, the arc reforms, the wires retouch and the process repeats.
When the arc is extinguished by the gas flow, i.e an arc outage, popping can occur. During an arc outage the wire continues to be fed and the ends remain melted to some extent. The partially melted wires then touch, restarting the arc. This problem may repeat in a pulsing manner in that after the arc is restarted the molten wire is blown away, and the arc is again extinguished.
If the arc spray is smooth (neither shortage nor outages occur) the resulting metal coating is repeatable and of acceptable quality. Spitting, popping and pulsing performance mean the resulting coatings may contain imperfections or not be repeatable, or both. Thus, it is desirable to detect the presence of both outages and shortages. As an electric arc spray operator becomes skilled, he develops both visual and audible perception of the process and develops a sense of when the spray plume is correct (running smooth) or not correct (spitting, popping and pulsing).
The visual and audible effects of the spray process depend on many factors, including the type and size of spray wires, the arc voltage and current and the gas flow rate. Thus, reading, or interpreting the visual and audible characteristics of the spray plume requires training of new operators by experienced operators and can take years to develop. Accordingly, it is desirable that an electric arc spray product detect and indicate the existence of wire shortages or arc outages so that an inexperienced operator may adjust the parameters if necessary, and also learn to recognize the characteristic sounds of these conditions under many varied uses.
To start the arc in an electric arc spray system the two wires are brought together until the metal to metal conduction of electrons begins. Since the touching resistance is low a large current flow through the wires will heat the wire ends until melting of the wire by the wattage produced. When current and voltage conditions are favorable, i.e. in the operating range where neither arc outages nor wire shortages occur, a smooth start of the spraying plasma is accomplished.
However, when the starting current and voltage is at or near wire shorting conditions, explosive starts can result because wire feed speeds are high. Such an initially high feed rate puts large amounts of unmelted wire in the arc area before sufficient heat can be developed. Frequently the leading end of the wires remains unmelted while a further back section will begin to melt and the magnetic field around the wire, due to the very high short current, will constrict this melted section. When the cross section of the melt becomes sufficiently small and its resulting electrical resistance very high, large amounts of wattage are produced very rapidly and an explosion occurs that expels the unmelted wire out of the gun and onto the surface to be coated.
Starting an arc at or near arc outage conditions requires very low wire feed rates (inches per minute, for example). Low feed rates cause the wires to just slightly touch and large amounts of heat to be generated instantaneously. The ends of the wire then burn off and the arc goes out. The whole process then starts over as the new wire ends again approach each other and lightly touch.
For the reasons described above, and to overcome motor inertia and other starting difficulties, starting the arc at or near either wire shortage or arc outage conditions areas is difficult. Accordingly when operation is desired to be at or near these conditions, an arc sprayer which starts the arc under normal operating conditions, and then moves to the difficult operating conditions, is desirable.